What is the minimum source-to-skin distance for a mobile x-ray unit?

• A. 6 inches
• B. 12 inches
• C. 18 inches
• D. 20 inches

Answer: (B)

The key idea is dose control when using a mobile X-ray unit. The dose the patient’s skin receives rises sharply as the distance between the X-ray source and the skin decreases, following the inverse square law. Because of this, there’s a safety baseline distance that must be kept to limit skin dose and make positioning practical for the operator. The minimum source-to-skin distance for a mobile X-ray unit is 12 inches (about 30 cm). This distance provides enough clearance to position the tube safely and still allow effective imaging without delivering excessive skin dose. Distances shorter than this would dramatically increase skin exposure, while larger distances are acceptable but exceed the minimum.